1. Field of the Invention
The invention relates to a copier, printer, facsimile machine or other image forming apparatus, and particularly to a charging device therefor. More specifically, the invention relates to an image forming apparatus which utilizes a charge roller for uniformly charging the surface of a photoconductive element, or image carrier, during a sequence of image forming steps.
2. Discussion of the Background
In a conventional image forming apparatus, a corona discharger is commonly utilized as a charging device for uniformly charging the surface of a photoconductive element or photoconductive member. A corona discharger effectively and uniformly charges the surface of a photoconductive element to a predetermined potential. However, corona dischargers are disadvantageous in that they require a high-tension power source and generate ozone during the discharge. Ozone generated in large quantities not only pollutes the environment but also accelerates deterioration of the charging member and the photoconductive element.
In view of the above problems, it has been proposed to utilize a charge roller in lieu of the corona discharger. With this type of charging device, a charge roller is held in contact with and driven by a photoconductive drum. The charge roller includes a magnetic core, and a voltage is applied from a power source to the core of the charge roller, such that the roller charges the surface of the drum. Using the charge roller, it is possible to lower the required voltage of the power source and reduce the amount of ozone ascribable to charging. In addition, the charge roller prevents dust particles from being electrostatically deposited on a corona wire, and eliminates the need for a high tension power source. However, the problem with this type of charger arrangement is that the charge distribution can become irregular, and the charge potential is extremely susceptible to environmental effects. In fact, such a charging arrangement is typically inferior to the corona charger arrangement with respect to the uniformity of the charge distribution.
Japanese Patent Laid-Open Publication No. 149668/1988 (referred to herein as the '668 system) teaches an arrangement to improve the uniformity of the charge superposing an AC voltage having a peak-to-peak voltage more than twice as high as a charge start voltage (Vth) in combination with the application of a DC voltage. However, the '668 system requires an AC power source in addition to a DC power source to thereby provide the superposed AC voltage together with the DC voltage, thus increasing the cost of the apparatus. Moreover, a great amount of AC current not contributing to the charge potential of the photoconductive element is consumed, thus reducing the efficiency of the apparatus. In addition to increasing the running and manufacturing cost of the apparatus, this arrangement also generates a great amount of ozone, which is problematic not only with respect to pollution of the environment, but also in aggravating or accelerating the deterioration of the charging member as well as the photoconductive element.
One type of charge roller to which only a DC voltage is applied includes an elastic layer formed of synthetic rubber with carbon dispersed therein. A problem with this type of charge roller (synthetic rubber with dispersed carbon) to which only a DC voltage is applied is that the resulting withstand voltage (i.e., the maximum voltage which the roller can withstand) of the charge roller tends to become weak.
In the conventional conductive elastic layer formed of a synthetic rubber and dispersed carbon, the electrical resistance is adjustable by varying the amount of added carbon to the synthetic rubber. However, it is difficult to satisfy both the electrical conductivity and the voltage requirements of the elastic layer at the same time. A typical electrical conductivity is a moderate 10.sup.8 .OMEGA.-cm. From a microscopic point of view, the electrical conductivity of carbon is quite different from that of the synthetic rubber. Therefore, an irregular charge distribution in the elastic layer of the charge roller occurs, and the withstand voltage of the elastic layer diminishes.
An additional problem with conventional charge rollers is that, during a stop mode or a non-operating period of time, conventional charge rollers are held in pressure contact with the photoconductive drum. As a result of this contact over a period of time, an abnormal image, such as an image with an undesirable lateral strip of toner or developer, is produced in the first copying operation (i.e., the first copy after an extended inoperative period). In addition, if the charge roller is in use over an extended period, undesirable toner becomes attached to the surface of the charge roller, and the capacity or ability of the charge roller to charge the photoconductive drum is thereby deteriorated.
Japanese Patent Laid-Open Publication No. 194061/1983 (referred to hereinafter as the '061 arrangement) teaches the use of a cleaning member for scraping undesired toner from the charge roller provided in the vicinity of the charge roller. In addition, a non-binding or non-adhering film is provided to avoid adherence of toner onto the surface of the charge roller. However, a problem with the '061 arrangement is that an irregular charge distribution results on the charge roller, and the level of the surface voltage of the charge roller is inadequate or less than desirable.
Japanese Patent Laid-Open Publication No. 222985/1990 (refereed to hereinafter as the '985 arrangement) discloses an electrophotographic apparatus which includes a photosensitive member and a charging member disposed in contact with the photosensitive member. The photosensitive member is charged by a voltage applied to the charging member. A ten-point mean (i.e., the average at ten location points) surface roughness (Rz1) of the photosensitive member and the ten-point mean surface roughness (Rz2) of the charging member satisfy the following relationships: EQU 0.1 micron&lt;Rz1+Rz2&lt;6.0 microns; EQU 0.05 micron&lt;Rz1&lt;5.0 microns;
and EQU 0.05 micron&lt;Rz2&lt;5.0 microns.
A rough surface is formed on the photosensitive member and the charging member by mechanical grinding to thereby lower the starting point or threshold point of discharge on the charging member. The capacity of the surface voltage of the charging member is thereby improved, and a more uniform charge distribution is attained as shown by the flatness of a surface voltage curve in a graphical representation of the surface voltage of the charging member. However, a problem with the '985 arrangement is that the non-binding or the non-adherence characteristics of the charging member with respect to the photosensitive member are not satisfactory, and the adherence of toner to the charging member is also not satisfactory.